When operating an extraction device having a circulating or reversing drive chain, it is enormously important to evaluate or monitor the chain condition of the drive chain and in particular the pre-tension of the chain by suitable measures. In principle, for this purpose optical, electrical, magnetic and mechanical sensor units, which are intended to supply measurement values from which the chain tension condition is to be derived and, using suitable algorithms, control parameters for the tensioning means or indeed also for the drive motors of the drive units are to be derived, have already been proposed.
DE 34 06 519 A1 proposes using a sensor unit which operates by magnetism to detect the chain tension magnetically in the case of tensionable drive stations. DE 34 06 519 proposes for this purpose a magnetic field generator which is associated with the respective run that comes from the drive station in the direction of movement of the chain. The magnetic field generator that is provided for measuring the chain tension in the upper run is arranged in the floor panel, and the magnetic field generator that is provided for measuring the chain tension in the lower run is arranged in the floor panel of the drive station above the lower run, in order to detect any alteration in the magnetic field that results from a change in the spacing between the drive chain that is to be measured and the links of the chain. In addition to hanging chain, the spacing between scrapers in a conveyor chain or indeed a change in the spacings between chain links is also to be established in combination with the regulation of hanging chain. The measuring principle in DE 34 06 519 is based on generating a directed magnetic field that is altered by the fact that chain links penetrate the magnetic field to different depths, wherein the alteration in the scanned magnetic field is to be evaluated by measuring technology. Since the magnetic transducer is arranged, with the measuring device, in the floor panel of the outgoing run, a large amount of wear is produced which only allows a short service life for a magnetic measuring system of this kind. It has not therefore been possible to develop the technical implementation of the system for detecting hanging chain and other chain conditions which is known from DE 34 06 519 A1 for mass production.